1. Field of the Invention
This invention relates to antennas, and more particularly, to antennas using holographic techniques for transmitting and receiving radiowaves.
2. Description of the Prior Art
Radio communications with an aircraft are plagued with dead spots in the radiation pattern. The design of the antenna on an aircraft always involves a compromise of weight and size to avoid interference with aircraft aerodynamics. For example, loop antennas are usually built into a pod that extends from the bottom of the aircraft presenting clearance problems. Rod antennas extend out from the skin of the aircraft much like a towel rod and are susceptible to breakage if icing is encountered. The prior art antennas that are appendages to the aircraft will result in blanking nulls which greatly reduces reception and transmission.
The present invention seeks to minimize the above concerns by providing an antenna that follows the surface configuration of the aircraft rather than being externally coupled to the aircraft. The antenna of the present invention utilizes holographic techniques to provide any desired radiation pattern or combination of patterns that are mounted to and part of the skin of the aircraft.
The use of holography in a manufacture of antennas is disclosed by Checcacci, et al. in an article entitled, "Holographic Antennas," published in Proceedings of IEEE, December 1968, pages 2165-2167. The interference pattern of a reference wave and a wave that provides the required radiation pattern is recorded from which a hologram can be constructed. A dipole antenna acts as the aperture of the interference absorbing screen. The hologram is constructed with variable attenuation or phase that has a profile equal to that of the recorded interference pattern (hologram). A major problem with the construction of the hologram is the elimination of the zero order wave.
A method of producing a hologram of a radio frequency source in space is disclosed in U.S. Pat. No. 3,488,656 issued to Anderson. A microwave source projects a plane wave toward the ground where a horn antenna is scanned along a series of straight lines detecting the incoming radiation of the microwave reference signal and the received radio frequency signal. The phase of the reference wave is compared with the phase of the radio frequency signal and the phase difference is recorded in the nature of an interference pattern. The recorded interference pattern is reduced in size on photographic film that can be viewed when illuminated by an RF source.
A satellite communication system using holographic techniques is disclosed in U.S Pat. No. 4,214,807 issued to Gfeller, et al. An optical transmitter directs a laser beam through a hologram to generate the desired multiple beam transmitter radiation pattern. The optical transmitter can be adjusted by providing several different holograms to generate different radiation patterns from the satellite. Each hologram is actuated by illuminating radiation from the transmitting laser which is selectively directed to the hologram at a particular angle. By defracting the laser radiations at the hologram, the wave fronts are reconstructed forming the desired field of the multi-beam radiation pattern. An optical antenna pattern of the optical transmitter is generated to transmit power to individual directional connections to selected earth stations.